Exploration Geochemistry

1
Exploration Geochemistry

• Christopher W. Klein
• GeothermEx, Inc.
• 5221 Central Ave. Suite 201
• Richmond, CA 94804

2
Topics

• Scope and Objectives of Exploration
• The System Types why Geochemistry?
• Importance of an Integrated Approach
• Choosing Tools Strategy
• Tactics Data Basics
• Water Tools
• Gas Tools
• Solids Tools
• Chemical Equilibrium Thermodynamics
• New Developments
• Data Management
• Further Information

3
1. Scope and Objectives of Exploration

• Given how poorly we understand so many geothermal
  systems, exploration encompasses almost all data
  gathering
• At the least
• Reconnaissance
• Pre-feasibility studies
• Feasibility studies
• Step-outs and field expansion during
  Development/Exploitation

The emphasis here
4

• Goals
• Commercial
• Academic/Scientific
• Blend
• Depends a lot on who is paying.

5
2. The System Types why Geochemistry?

• Volcanic - magmatic
• Andesitic / Island Arc
• Basaltic / Oceanic Ridge - Hawaiian
• Silicic / Continental (Calderas)
• Deep Sedimentary Trough / Spreading Center
• Continental Heat-Flow
• Basin and Range (Extension/ high regional H-F)
• Background H-F
• Chemical/Phase Type
• Liquid-dominated
• Two-phase
• Steam-dominated
• Altered meteoric water
• Altered seawater

Basic Manifestations Waters - springs,
wells Gases - fumaroles, springs,
wells Hydrothermal Alteration
6
3. Importance of an Integrated Approach

• Dont limit the geochemical point-of-view to one
  discipline if others may be relevant
• Conclusions must be reasonable in light of other
  data and information
• Geology
• Temperature
• Well data
• Geophysics

7
4. Choosing Tools Strategy

• Commercial viewpoint
• Try to avoid discovering what you already know,
  or more than you need to know.
• Does the proposed study have a reasonable chance
  of assisting a project decision (resource
  assessment / drilling / finance / etc.) in a way
  that other information could not?

8
5. Tactics Data Basics

• Too much data rarely the problem
• Wrong data can be a problem
• Thorough and disciplined record-keeping
• Location, location, location
• GPS
• Maps of results and synthesis of data at common
  scale
• Contours drawn by hand (not by computer)
• Quality control
• During data gathering/generation
• During data analysis
• Data management

9
EXPLORATION TOOLS
10
6. WATER TOOLS

• The H2O itself
• Isotopes
• Phases (liquid / vapor)
• Whats in it solutes / gases
• Chemistry
• Isotopes

11
STABLE ISOTOPES OF WATER
dD or d18O 1000 (Rsample Rstd)/Rstd
(permil or o/oo) So Seawater dD 0 o/oo and
d18O 0 o/oo dD or d18O lt 0 lighter dD or
d18O lt 0 heavier H216O is about 10 lighter
than H218O, and chemically more reactive
12
(No Transcript)
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
Radioisotopes of Water
1 Tritium Unit (TU) 1 atom 3H per 1018 atoms
1H Before 1953 atmospheric TU 3-5 By 1963
atmosphere at several 1000 TU European atmosphere
now lt10 TU Groundwater gt30 TU implies recharge
in 1960s lt1 TU implies older
17
Solutes Major Anions
Chloride 50 to 20,000 mg/kg (to 200,000 mg/kg
in hypersaline brines) Sources traces of Na-K-Cl
in volcanic rocks (seawater origins), connate
seawater in sedimentary rocks, halite deposits
Bicarbonate lt1 to several 1000 mg/kg (for most
purposes, effectively the same as
alkalinity) Sources reactions of dissolved CO2
from atmosphere and/or in geothermal/volcanic
steam, with silicate minerals in rocks, with
carbonate minerals (limestone)
seawater Cl 19,350 mg/kg
Sulfate 10 to 1500 mg/kg (to 100,000 mg/kg in
acid volcanic steam condensates Sources oxidized
sulfide minerals and H2S, sulfate mineral
deposits (gypsum, anhydrite)
Approximate range among non-volcanic geothermal
systems (higher SO4 exist)
Extremes of volcanic and steam heated are acidic
(no HCO3)
18
Solutes Major Anions and Cations
3
1
3 component mixing
1
1
1
1
2
2
19
3
Synthesis of Results component origins on a map
2
2
1
20
Tri-linear diagrams can be made using any three
components
Schoeller (spider) diagrams can illustrate entire
analyses
Log (concentration)
Species (Na, K, Ca, etc.)
Source Giggenbach (1991)
21
Mixing diagrams can be constructed comparing
dissolved species to enthalpy (temperature) Chlor
ide ion is best for this, because it does not
participate in chemical reactions. Other
conservative or nearly conservative species
(aqueous tracers) B, Li, Rb, Cs, Br, the stable
isotopes of water.
22
Chemical Geothermometers Rely upon chemical
species (solutes, gases, isotopes) reaching a
state of reaction equilibrium in the reservoir,
then leaving the reservoir and appearing at
wells/springs/fumaroles before significant
re-equilibration can occur.
Qualitative comparison of reaction times (Henley
and others, 1984)
e.g. reactions that control pH, Carbonate
deposition
23
Silica Geothermometers
24
Silica The Chalcedony Quartz Problem
Data from geothermal wells in Nevada
25
Silica Salinity Effects - 1
26
Silica Salinity Effects - 2
27
Cation Geothermometers - 1

• Na/K - Ion exchange in alkali feldspars (common
  in volcanic rocks) causes Na/K to decrease as T
  increases.

• Simple plots of K vs Na can be a guide to
  relative source temperatures.
• Considered applicable only at gt150C.
• Clay mineral interference at lt200C can yield
  temperatures that are too high.
• Various calibrations available (Fournier,
  Giggenbach, Truesdell, Arnorsson)

28
Cation Geothermometers - 2

• Na-K-Ca Developed and calibrated by Fournier
  and Truesdell (1973).
• Empirical, but based on a theoretical
  consideration of likely silicate reactions, to
  incorporate the influence of calcium-bearing
  minerals (feldspars, epidote, calcite)
• Considered more acceptable than Na/K over
  100-300C
• High Pco2 at low temperature yields poor results
  due to high Ca. Pco2 correction can be applied.
• Eqn has two forms the correct one needs to be
  applied (depends on TC, Ca, Na)
• Other versions available Benjamin and others,
  1983 illite form of Ballantyne and Moore, 1990)

29
Cation Geothermometers - 3

• Lower-T waters and shallow-cooled reservoir
  zones if Mg gt1 ppm.
• Na-K-Ca-Mg Applies correction to Na-K-Ca.
  Developed and calibrated by Fournier and Potter
  (1978)
• K-Mg Developed by Giggenbach, alternate
  calibrations by Fournier, Arnorsson

30
Effects of Reservoir Cooling Silica, Na/K and
Na-K-Ca geothermometers All wells are within a
single geothermal field in Nevada, USA
31
Effects of Reservoir Cooling K-Mg and Na/K
geo-thermometers Calibrations by Giggenbach,
Fournier, Arnorsson
32
Other Aqueous Geothermometers

• Sulfate-Water Oxygen Isotope re-equilibrates
  very slowly with cooling, may be very accurate if
  SO4 not added/removed (mixing, anhydrite/gypsum)
• Anhydrite equilibrium (CaSO4) Accuracy depends
  upon thermodynamic data for the equilibrium
  reaction.
• K-Mg-Ca (Giggenbach) simultaneous T dependence
  of K2/Ca and K2/Mg (reactions involving
  feldspars, mica, Ca-Al-silicate, calcite, CO2,
  chlorite)
• Na/Li and other ion ratios rarely used.

33
Mathematical Mixing Models
Example Nevis, W.I., 55C submarine spring Cl
at 16,400 mg/kg (thermal water contaminated by
seawater).
Process remove seawater to the point where the
thermal component contains 1 mg/kg of Mg. Result
thermal Cl at 11,000 mg/kg, geothermometers
converging at 175C
175C
Chemical Temperature (C)
Fraction seawater in sample
34
Other Water Parameters (Less Widely Used)

• To distinguish provenance
• Isotopes of C, S, B, Cl
• Rare earth elements, Y
• Isotope geothermometers (gaswater, gas-gas)
• 18O H2O CO2
• 2H H2 H2O, H2 CH4, H2O CH4
• 13C CO2 CH4, CO2 HCO3
• 34S SO4 H2S

35
7. Gas Tools

• Advantages at volcanic systems
• more fumaroles/seeps than springs
• fumaroles usually above reservoir (short pathway
  to surface)
• Limitations
• minor to insignificant in outflow zones and in
  non-volcanic settings
• chemistry more complex than water
• greater difficulty and expense to sample

36
Significant Gas Components

• Relatively more soluble in water
• NH3, H2S, CO2
• Relatively less soluble
• CH4, H2, N2, Ar, He, (other noble gases)
• Higher T systems significant CO2, CH4, H2
• Lower T systems dominated by N2
• Volcanic/magmatic SO2, HCl, HF
• Measurable O2 indicates contamination by air from
  shallow source or during sampling.

37
As with solutes in water, any three gas
components can be combined in a tri-linear
diagram
An alternate view puts He (which comes from
radioactive decay in the earths crust) at this
apex.
CH4 H2S CO2 can be useful to show boiling
trends
38
Gas Geothermometry - 1

• Empirical
• determined for studied areas (e.g. Iceland)
• best fits of data to source temperature
• Theoretical / thermodynamic
• based on chemical reactions, some with minerals,
  assuming equilibrium
• Major ambiguity - whether gases sampled originate
  from reservoir steam, boiling of liquid, or both.

39
Gas Geothermometry - 2

• Giggenbach Gas Ratio Grids thermodynamic basis,
  with simplifying assumptions
• Example H2/Ar vs. CO2/Ar
• Others
• H2/Ar vs. T
• CH4/CO2 vs. CO/CO2
• CO/CO2 vs H2/Ar

40
Other Gas Parameters
3He/4He magmatic (high) vs. crustal (low) (3He
mantle source 4He decay of U, Th,
Ar) 40Ar/36Ar atmospheric (low) vs. magmatic
(high) Noble gas ratios (various) Stable
isotopes of steam condensate

41
8. Solids Tools

• Hydrothermal Alteration Maps
• Can outline extent of reservoir
• Fluid type(s) responsible
• Temperature(s) of alteration
• Limitation may indicate paleo-conditions only
• Fluid Inclusion Analysis
• Leakage Detection Surveys (faults/fractures)
• Soil gas Hg, Rn, CO2
• Soil ammonia, Sb, As, B, Hg, Gamma
• Evidence of reservoir cap rock (clay minerals)
• May assist resistivity survey interpretation

42
9. Chemical Equilibrium Thermodynamics

• Calculate simultaneous chemical reaction states
  in a large suite of dissolved and solid species
• Requires good data (esp. pH, alkalinity /
  bicarbonate, Al)
• Useful for geothermometry, mixing, precipitation
  and dissolution of solids
• Some thermodynamic data are uncertain
• Available codes differ in capabilities

43
(No Transcript)
44
10. New Developments

• Software and Equipment
• Database software
• Graphing software
• In the field GPS
• High Performance/Pressure Liquid Chromatography
  better anion data, esp. SO4
• Methods
• More common/refined use of AA for SiO2
• Biggest Downer increased difficulty of shipping
  samples, esp. gases

45
11. Data Management

• Spreadsheets
• Convenient for smaller amounts of data
• Lead to sloppy/inconsistent formatting
• Limited input/edit forms screen capability
• Calculations may contain hidden errors
• Graphing can suffer from inadequate format
  control
• Databases
• Better for data sets with gt2540 analyses
• Enforce discipline in formatting
• Unlimited input/edit forms screen capability
• Calculations are external to the data
• Use separate graphing package

46
(No Transcript)
47
12. Further Information

• Arnórsson, S., 2000. Isotopic and Chemical
  Techniques in Geothermal Exploration, Development
  and Use Sampling Methods, Data Handling,
  Interpretation. International Atomic Energy
  Agency, Vienna
• Bethke, C.M., 1996. Geochemical Reaction
  Modeling, Concepts and Applications. Oxford
  University Press, New York, Oxford.
• DAmore, F. (Co-ordinator), 1991. Applications
  of Geochemistry in Geothermal Reservoir
  Development. Series of Technical Guides on the
  Use of Geothermal Energy. UNITAR/UNDP Centre on
  Small Energy Resources, Rome Italy.
• Ellis, A.J. and W.A.J. Mahon, 1977. Chemistry
  and Geothermal Systems. Academic Press.
• Henley, R.W., Truesdell, A.H. and Barton, P.B.,
  1984. Fluid-Mineral Equilibria in Hydrothermal
  Systems Reviews in Economic Geology, Vol. 1,
  Society of Economic Geologists, Univ. Texas, El
  Paso, TX
• Hem, J.D., 1989. Study and Interpretation of the
  Chemical Characteristics of Natural Water.
  United States Geological Survey Water-Supply
  Paper 2254.
• Nicholson, K., 1993. Geothermal Fluids
  Chemistry and Exploration Techniques.
  Springer-Verlag.
• The Encyclopedia of Water Environmental Isotopes
  in Hydrology (at www.wileywater.com)